I've looked into the skill increase / attribute increase more, and now feel I had a wrong expectation of it.
The terms ncgdmw uses for attribute growth is misleading.
fast is close to vanilla morrowind leveling system and should probably be called normal.
For standard and slow hard & very hard would be a lot more appropriate.

I also tested the code change i posted in a bug report on nexus, https://www.nexusmods.com/morrowind/mods/44967/.
It adjusts the stored values when ncgdmw sees the 2 strength increase from returning cloudcleaver and I didn't find any problems with it.
If someone wants my edited version of ncgdmw, just let me know.

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Falc, I looked at ncgdmw code and think the error you got may be a symptom of a bigger problem.
Afaics the number subtracted from starting attribute values is 25 , and that is only at initialization.

With a normal start 30 is lowest possible starting value before ncgdmw kicks in.
Even if all skills started at 0, no stored value should be lower then 5 .
Damage and drain attribute effects should have no influence on those stored vars.

For information this mod does not work with quick or alternative starts, at best you get the error Falc reported usually you die immediately.

I rewrote most of this mod as I start with altered racial bonuses and birthsigns and radically reduced attributes and skills. Normally you have no start attribute over 25 and no skill over 15. I removed the code for choosing growth rates, you are stuck with what you call very hard. It takes about 3 days game time to see any increases and do not think about getting into a serious fight for a couple of weeks, even mud crabs are dangerous. But this is how I prefer to play.

I've looked into the BTB charater plugin and there are 2 potential conflicts with ncgdmw in it.

First is the disabling of birthsign stat boosters when sleeping (designed to make sure the boosters don't limit max attribute to 100 upon levelling)
ncgdmw doesn't use the morrowind leveling at all, but has it's own system to allow attributes to go over 100.
BTB used fortify attributes / fortify skill spells for the boosters that should work fine with ncgdmw .

second are the changes btb made to starting values :
All races except dark elves/ imperial / argonian have atleast one attribute value that starts at 20 .
ncgdmw subtracts 25 from all starting attributes so the attributes that start at 20 become -5 .

I doubt very much openmw / morrrowind allow negative values for attributes and change them to 0 .

Either BTB character plugin or ncgdmw need to change to prevent this.

The simplest solution would probably be an edited version of btb character plugin with 25 instead fo 20 as lowest racial value.
Doing it that way imo stays close to what BTB and greywander wanted to achieve with their plugins.